The long-term goal of this research is to understand the molecular basis of insecticide resistance in arthropod pests. As carriers of numerous human pathogens and/or a major source of indoor allergens, arthropod pests, such as cockroaches, mosquitoes and ticks, are major threats to human health. Strategies for the control of these arthropod pests rely heavily on the use of insecticides, such as pyrethroids which act on voltage-gated sodium channels. However, the intensive use of pyrethroids has led to rapid development of insecticide resistance worldwide. One major mechanism of resistance, knockdown resistance (kdr), reduces neuronal sensitivity and confers cross-resistance to all pyrethroid insecticides. Research in the past decade shows that mutations in the sodium channel gene are responsible for kdr in various arthropod pest species. Our recent findings indicate that there are two distinct mechanisms of pyrethroid resistance: one via reducing pyrethoid-binding and the other via altering gating properties. These results set a critical stage for hypothesis-driven research to elucidate the pyrethroid-binding site and how alterations of gating properties of sodium channels cause pyrethroid resistance. We have also begun to study the molecular action of a newly developed insecticide, indoxacarb, that acts on the sodium channel by a mechanism completely different from that of pyrethroids. The central hypothesis to be examined in this proposal is that these two classes of insecticides bind to distinct receptor sites on the sodium channel and trap distinct voltage-sensing domains in the outward configuration, modifying sodium channel function. Pyrethroids interact mainly with domain II of sodium channels, inhibiting channel deactivation, whereas indoxacarb interacts mainly with domain IV, promoting channel inactivation. The three specific aims of this proposal are: 1. Comprehensive analysis of the molecular determinants of pyrethroid-binding based on naturally occurring kdr mutations and site-directed mutagenesis. 2. Determine the molecular basis of pyrethroid resistance caused by alterations in sodium channel gating properties. 3. Characterize the molecular action of indoxacarb on the insect sodium channel.